The propensity of the rat to gnaw must not be attributed altogether to a reckless determination to overcome impediments. The never-ceasing action of his teeth is not a pastime, but a necessity of his existence. The writer of an interesting paper on rats in _Bentley's Miscellany_ has explained so clearly the dentistry of the tribe, that we extract his account:--
"The rat has formidable weapons in the shape of four small, long, and very sharp teeth, two of which are in the upper and two in the lower jaw. These are formed in the shape of a wedge, and by the following wonderful provision of nature have always a fine, sharp, cutting edge. On examining them carefully, we find that the inner part is of a soft, ivory-like composition, which may be easily worn away, whereas the outside is composed of a glass-like enamel, which is excessively hard. The upper teeth work exactly into the under, so that the centres of the opposed teeth meet exactly in the act of gnawing; the soft part is thus being perpetually worn away, while the hard part keeps a sharp, chisel-like edge; at the same time the teeth grow up from the bottom, so that as they wear away a fresh supply is ready. The consequence of this arrangement is, that, if one of the teeth be removed, either by accident or on purpose, the opposed tooth will continue to grow upwards, and, as there is nothing to grind them away, will project from the mouth and turn upon itself; or, if it be an under-tooth, it will even run into the skull above. There is a preparation in the museum of the Royal College of Surgeons which well illustrates this fact. It is an incisor tooth of a rat, which, from the cause above mentioned, has increased its growth upwards to such a degree, that it has formed a complete circle and a segment of another; the diameter of it is about large enough to admit a good-sized thumb. It is accompanied by the following memorandum, addressed by a Spanish priest to Sir J. Banks, who presented it to the Museum: 'I send you an extraordinary tooth of a rat. Believe me, it was found in the Nazareth garden (to which order I belong). I was present when the animal was killed, and took the tooth; I know not its virtues, nor have the natives discovered them.'"